In more detail
The period of a wave is the reciprocal of its frequency, T = 1 / f: 50 Hz mains repeats every 20 ms and 60 Hz every 16.7 ms. Larger units cover radio and electronics: kilohertz, megahertz and gigahertz. In music, the note A above middle C is 440 Hz, each semitone multiplies the frequency by 2^(1/12) ≈ 1.0595 and each octave doubles it, so middle C is about 261.63 Hz. Wavelength is speed divided by frequency: a 440 Hz tone in air (343 m/s) is 0.78 m long.
Why it matters
Humans hear roughly 20 Hz to 20 kHz, with the upper limit falling with age, and speakers or headphones can be tested with generated tones across that range. Display refresh rates (60, 120, 144 Hz) set how often the picture updates, and mains frequency differs by region (50 Hz in most of Europe, 60 Hz in North America), which matters for some clocks and motors.
Example
440 Hz: T = 1 / 440 = 2.27 ms
Middle C: 440 × 2^(−9/12) = 261.63 Hz; one octave up = 523.25 Hz
144 Hz display: 1 / 144 = 6.94 ms per frame
Common confusion
Sample rate is also given in hertz, but it counts samples per second, not cycles of a wave: 44.1 kHz audio can represent frequencies only up to 22.05 kHz, half the sample rate. Music tempo is in BPM (beats per minute); divide by 60 for hertz.